1. Field of the Invention
The present invention relates to a sheet stacking device equipped with a buffering function and an image forming apparatus provided with the sheet stacking device. Here, an image forming apparatus according to the present invention signifies apparatuses including a sheet stacking device and an image forming apparatus main body.
2. Related Background Art
Conventional image forming apparatuses such as a copying machine include an apparatus in which a sheet having an image formed in an image forming unit is subjected to post-processing. The post-processing is exemplified by that a sheet having an image formed thereon is temporarily stacked on a intermediate-processing tray, followed by stapling processing, before a set of sheets is stacked on a stack tray.
In the case where the stapling processing is performed, a sheet discharged from the image forming apparatus is first stacked on the intermediate-processing tray, followed by the stapling processing after all of the sheets are stacked on the intermediate-processing tray. The set of sheets subjected to the stapling processing is discharged to the stack tray, and then, a first sheet out of a next set of sheets is stacked on the intermediate-processing tray.
Here, in comparison of a time of period during which the set of sheets is stacked on the intermediate-processing tray in the case where the stapling processing is performed with a time of period in the case where no stapling processing is performed, the time of period in the case where the stapling processing is performed becomes longer by the time of period taken for the stapling processing.
In other words, a sheet for a next job cannot be stacked on the intermediate-processing tray during the stapling processing. In view of this, in the case of the continuous stapling processing, an interval between the last sheet of the preceding set of sheets and the first sheet of the following set of sheets is lengthened by delaying the feeding of a first sheet for the next job. As a consequence, the productivity at the time of the stapling processing is degraded more than that at the time of no stapling processing.
In order to solve the above-described problem, there has been conventionally proposed and provided an apparatus equipped with a buffering function of allowing a sheet being conveyed to temporarily stay so as to stack the sheet on a intermediate-processing tray with a delay while keeping the productivity of image formation (see Japanese Patent Application No. 2000-352923). However, a space is required for disposing a special buffer path inside of the apparatus, thereby making it difficult to reduce the size of the apparatus. In contrast, although the above-described configuration may be achieved if a sheet stacking device is installed outside of the image forming apparatus, even in this case, it has been difficult to incorporate the sheet stacking device provided with the buffer path inside of the image forming apparatus.
In addition, in the case where the sheets subjected to buffering processing are discharged to the intermediate-processing tray in superimposition, it has been difficult to align the sheets in superimposition, in particular, to secure the alignment of the sheets stacked downward.